1. Field of the Invention
The present invention relates to a process for making esters. More particularly, it relates to a process for making fatty acid alkyl esters.
2. Brief Description of Related Art
Bio-diesel typically include long chain, fatty acid alkyl esters produced from vegetable oils or animal fats by transesterification of the triglycerides with lower alcohols (for example, methanol and/or ethanol). Bio-lubricants may be prepared through transesterification of triglycerides with C5 to C12 alcohols. In recent years, there is a growing tendency towards using vegetable-based products in many formulations as substitutes for the petroleum-based products. This prompts researchers that are more concerned about environmental protection and greenhouse gas effects to develop bio-based products. These new bio-based products derived from vegetable and plant products, such as soybean, sunflower, and rapeseed etc, are renewable, biodegradable, less environmental hazardous, and safer to handle. Similarly, other renewable sources of triglycerides include rendered animal fats and waste cooking oils from commercial food production. Rendered animal fats and waste cooking oils may be sulfur-free and are used in automobile applications, mechanical engine applications, cosmetic applications, and soaps.
The heating value of vegetable oil is similar to that of fossil fuel (for example, diesel), but the direct use of vegetable oils in the diesel engines is limited by some of their physical properties. For example, the viscosity of vegetable oil is about 10 times the viscosity of diesel fuel. Transesterification is one among the four major available treatments, which is most commonly employed to improve the fuel value and lubricant utility of triglycerides.
Several processes for the transesterification of triglycerides have been developed including the acid and base-catalyzed, homogeneous processes. Base-catalyzed reactions are simple and reasonably economical and, in fact, are used commercially in several countries for bio-diesel production and are described in J. Brazil. Chem. Soc. Vol. 9, Year 1998, pages 199 to 210; Bioresource Tech. Vol. 70, 1999, pages 1 to 15; Catalysis Vol. 19, 2006, pages 41 to 83; J. Am. Oil Chem. Soc. Vol. 75, 1998, pages 1775 to 1783; and U.S. Pat. Nos. 6,015,440 to Noureddini and 6,489,496 to Barnhorst et al. all of which are incorporated herein by reference. The use of alkali metal alkoxides may result in high yields of fatty acid alkyl esters in a short reaction time, however, alkali metal alkoxides may cause corrosion of components used in preparing the fatty acid alkyl acid esters. In addition to corrosion problems, these homogeneous catalyst-based processes involve elaborate process steps for removal of free fatty acids (“FFAs”) and water from the feedstock and catalyst from the products. When FFAs are present, they react with the homogeneous alkali catalysts, form unwanted soap by-products, and deactivate the catalyst. In addition, water, sometimes present in non-edible, unrefined or waste vegetable oils also leads to the deactivation of homogeneous catalysts as described in Bioresource Tech. Vol. 70, 1999, pages 1 to 15. Separation and reuse of the homogeneous catalyst system is an issue needing additional process steps.
Several heterogeneous base and acid catalyst systems like metal oxides of tin, magnesium and zinc, Cs-MCM-41, Cs-exchanged NaX, ETS-10, Mg/Al hydrotalcites, alkali nitrate and alkali carbonate-loaded Al2O3, polymer resins, sulfated-tin and zirconia oxides and tungstated-zirconia have been reported for the transesterification of vegetable oil with alcohols are described in J. Am. Oil. Chem. Soc. Vol 78, 2001, page 1161; Bioresource. Technol. Vol. 70, 1999, pages 249 to 253; Appl. Catal. A: Gen. Vol. 218, 2001, pages 1 to 11; Catal. Today Vol. 93-95, 2004, pages 315 to 320; Green Chem. Vol. 6, 2004, pages 335 to 340; J. Mol. Catal. A: Chem. Vol. 109, 1996, pages 37 to 44;Appl. Catal. A: Gen. Vol. 257, 2004, pages 213 to 223; Ind. Eng. Chem. Res. Vol. 44, 2005, pages 7978 to 7082; Adv. Synth. Catal. Vol. 348, 2006, pages 75 to 81; Appl. Catal. A: Gen. Vol. 295, 2005, pages 97 to 105; J. Catal. Vol. 229, 2005, pages 365 to 373, all of which are incorporated herein by reference. Leaching of metal ions is encountered in a significant number of these heterogeneous systems. Free fatty acids, when present, inhibit the transesterification on solid basic catalysts, and, thereby, confine the catalyst systems to the transesterification of edible oils. Additional pre-treatment process steps are needed when using non-edible oils.
U.S. Pat. Nos. 5,908,946 to Stern; 6,147,196 to Stem et al.; and 6,878,837 to Bourna et al., all of which are incorporated herein by reference, describe the production of alkyl esters from vegetable and animal fat, and an aliphatic mono-alcohol in the presence of a heterogeneous zinc aluminate catalyst. Water inhibits this catalyst system and its presence in the reaction medium beyond an amount of 1000 ppm by weight is undesirable. U.S. Pat. No. 6,960,672 to Nakayama et al., which is incorporated herein by reference, describes the application of a catalyst that includes a composite metal oxide having a perovskite structure. The reaction was conducted by making alcohol into a supercritical state or subcritical state. U.S. Pat. No. 5,525,126 to Basu et al., which is incorporated herein by reference, describes the application of a mixture of calcium acetate and barium acetate. At the reaction conditions some amount of metal leaches into the liquid portion, hence, it is not a truly heterogeneously catalyzed system. U.S. Pat. No. 7,122,688 to Lin et al., which is incorporated herein by reference, describes the application of acidic mesoporous silicate for producing bio-diesel.
U.S. Pat. No. 5,713,965 to Foglia et al, which is incorporated herein by reference, describes the production of bio-diesel, lubricants and fuel and lubricant additives by transesterification of triglycerides with short chain alcohols in the presence of an organic solvent such as an alkane, an arene, a chlorinated solvent, or a petroleum ether using Mucor miehei or Candida Antarctica-derived lipase catalyst. International Patent Application Publication Nos. WO 00/05327 to Gnosar et al., WO 02/28811 to Koncar, WO 2004/048311 Muskett et al., WO 2005/021697 to Oku et al., and WO 2005/016560 to Hooker; and U.S. Pat. Nos. 5,578,090 to Bradin; 6,855,838 to Haas et al.; 6,822,105 to Luxen et al.; 6,768,015 to Luxem et al.; 6,712,867 to Boocock; 6,642,399 to Boocock; 6,399,800 to Haas et al.; 6,398,707 to Wu et al.; 6,015,440 to Noureddini, all of which are incorporated herein by reference, describe the production of fatty acid alkyl esters using either lipase catalysts or metal ion catalysts. International Patent Application Publication No. WO 2004/085583 describes transesterification of fats with methanol and ethanol in the presence of a solid acid catalyst having ultra-strong acid properties in a short time at around ambient pressure.
Production of diesel from pure soybean oil or coconut oil is not economical, so it is desirable to use cheaper alternative feedstock such as animal fat or used cooked oil or oil from seeds of wild plants like jojoba, jatropha or karanja. Animal fat and used oil contain high amounts of FFAs content. The FFAs saponifies with the alkali-based transesterification catalyst leading to low yield, difficulties in separation of the products, and increase in production cost. In those cases, a two step process in which during the first step an acid catalyst esterifies the free fatty acids to methyl esters and in the second step, transesterification of the triglycerides over a base catalyst is generally employed in diesel preparation.
Many methods and/or catalyst for the transesterification of fatty alkyl acids have been proposed, however, many conventional catalysts loose their activity on recycle and/or require pretreatment of the feedstock to remove the free fatty acids and water. Some conventional catalysts require harsh reaction conditions. Hence, an efficient, highly active catalyst capable of transesterifying both edible and non-edible vegetable oils in refined or unrefined forms at mild conditions is highly desirable. Such a catalyst system enables economic benefits and makes the bio-diesel and bio-lubricants an economical alternative to petroleum based diesel and lubricants.